1. Field of the Invention
The present invention relates to a chuck for holding a rotary cutting tool such as a drill and, more particularly, to a chuck wherein a clamping cylinder is mounted directly or through needle rollers on a holding cylinder having a rotary cutting tool or an adapter for holding such a tool inserted therein, and in which a contracting or recovery force acts on the holding cylinder when the clamping cylinder rotates.
2. Description of the Prior Art
A roll lock-type chuck is known as a chuck for holding a rotary cutting tool as per Japanese Utility Model Publication No. 41-23987. In this chuck, a clamping cylinder is rotatably mounted at a predetermined distance around a holding cylinder in which an objective tool, e.g., a cutting tool, is directly or indirectly inserted. A number of needle rollers are arranged at a plurality of stages along the entire circumference of a ring-like gap or space formed between the two cylinders. The needle rollers are parallel to each other and inclined at predetermined angles with respect to the common axis of the cylinders. When the clamping cylinder is rotated, the needle rollers rotate while they revolve around the axis of the cylinders so as to contract or recover the holding cylinder. In a chuck of this type, since the clamping cylinder is rotated through the needle rollers, friction resistance is small and contraction/recovery of the holding cylinder is easy when compared to conventional chucks.
To the contrary, in a chuck of this type, since the holding cylinder is thin in order to provide a large amount of contraction (deformation amount), its strength is small. Thus, when heavy cutting must be performed, the chuck elastically deforms and the total oscillates.
In addition, in a chuck of the type described above, the holding cylinder and the shank of the cutting tool are in contact with each other along their entire circumferential surfaces. For this reason, when contraction/recovery is performed before cutting, an oil film is formed on the inner circumferential surface (surface holding the shank of the cutting tool) of the holding cylinder, decreasing the holding force. This may cause slippage of the tool or may not allow cutting at all.
In order to solve the above problems, a plurality of slits can be formed in the holding cylinder at predetermined intervals along the circumference so as to extend along the axial direction of the holding cylinder. This may facilitate contraction of the holding cylinder and prevent slippage of the tool. However, when such slits are formed, one end of each needle roller, rolling upon rotation of the clamping cylinder, drops into a slit. Thus, the rotation of the holding cylinder is interfered with, and smooth rotation and revolution of the needle rollers cannot be obtained.
In view of this, a technique is proposed in Japanese Utility Model Publication No. 59-33534 (to be referred to as the first technique hereinafter). In this technique, a thin holding cylinder (outer chuck cylinder) is used. An inner chuck cylinder having a plurality of slits at predetermined intervals along the circumference is inserted in the inner circumferential surface of the holding cylinder. The shank of the cutting tool is held with this inner chuck cylinder.
In the first technique, since a plurality of slits are formed in the inner chuck cylinder, the inner chuck cylinder can be deformed easily. However, since an outer chuck cylinder must also be included, the number of parts is increased. Thus, the chuck becomes large in size and heavy in weight. In addition, since the inner and outer chuck cylinders must be fixed concentrically, the overall cost of the chuck is increased, the machining precision of parts is lowered, and assembly errors are increased.
In order to solve these problems with the first technique and to prevent oscillation and slippage of the tool during cutting, other techniques have also been proposed in Japanese Utility Model Disclosure No. 52-23774 and Japanese Utility Model Publication No. 57-33930 (to be referred to as second and third techniques hereinafter). In the second technique, recessed portions, such as grooves, are formed at predetermined intervals along the inner circumferential surface of the holding cylinder so as not to extend to the outer circumferential surface. In the third technique, deep holes are formed in an end face of the holding cylinder.
In the second technique, since recessed portions, such as grooves, are formed in the inner circumferential surface of the holding cylinder, the contact area with the tool shank is reduced and an increased holding force can be obtained. However, since the distal end of each recessed portion is enlarged in a sector shape during contraction of the holding cylinder, pointed edges are formed at the distal ends of the recessed portions and are pressed into the tool shank. The pointed edges thus cause damage to the shank. Once damaged, the tool shank must be repaired before it can be used again.
Formation of recessed portions such as grooves in the inner circumferential surface of the holding cylinder alone does not solve the problems.
In the third technique, the problem of contact between the entire surfaces of the holding cylinder and the tool shank is also not solved. In addition, in order to form deep holes in an end face, the holding cylinder necessarily becomes thick. This results in a large, heavy chuck. Moreover, since a thick holding cylinder is used, a large force must be applied to deform it. Slippage of the tool cannot be prevented.